Shark - biology. I INTRODUCTION Shark, any of several fast-swimming, generally large fish known for their many sharp teeth, distinctive dorsal fin, and skill in locating underwater prey. Like other fishes, sharks are generally cold-blooded and breathe with gills. But unlike the more common bony fishes, which have skeletons made of rigid bone, sharks--and their relatives skates, rays, and chimaeras--have flexible internal skeletons made of cartilage. Among the most ancient vertebrates in the sea, ancestors of modern sharks originated almost 400 million years ago. Today sharks live in essentially the same way they did more than 200 million years ago, before the rise of the dinosaurs. Scientists have identified nearly 375 species of sharks living today, ranging in size from the dwarf dogfish, less than 20 cm (8 in) in length, to the massive whale shark, which reaches lengths of more than 15 m (50 ft) . Most sharks inhabit tropic or temperate marine waters, but some species have been found in polar seas. The bull shark leaves ocean waters to enter freshwater rivers and lakes, including the Zambezi River in southeastern Africa, the Mississippi River in the United States, and Lake Nicaragua in southwestern Nicaragua. Depending on the species, sharks inhabit either shallow coastal waters or the open ocean. Some species, such as the sixgill shark, live at depths of more than 1,800 m (6,000 ft). No matter where they live, sharks play a crucial role in their ecosystems. As highly efficient predators, sharks keep ecosystem populations in check by hunting and killing other animals, particularly those that are weak or diseased. Biologists fear that without sharks, populations of other animals would swell to unsustainable levels. These animals would eliminate the available food in their ecosystems, leading to massive die-offs from starvation. It might take years, or even decades, for the organisms in the ecosystem to again achieve the delicate balance that sharks help maintain. II PHYSICAL DESCRIPTION Sharks have streamlined bodies that permit short bursts of movement through the water at high speeds. Their long bodies taper into a blunt or slightly pointed snout on one end and a powerful tail fin on the other. This shape reduces drag in the water, enabling sharks to glide gracefully, expending minimal energy as they swim. Generally, sharks swim at speeds less than 5 km/h (3 mph), but many can reach speeds of more than 20 km/h (12 mph) in short bursts. Mako sharks, swift, powerful, open-ocean sharks that reach lengths of up to 3.5 m (12 ft), are believed capable of speeds up to 48 km/h (30 mph). Although some sharks grow to more than 15 m (50 ft) in length, massive size in sharks is the exception rather than the rule. About half of all known species never grow to more than 1 m (3 ft) in length, and less than 20 percent reach lengths longer than 2 m (6 ft). Some of the best-known sharks, however, are the largest sharks. Nurse sharks, slow-moving sharks that live mostly in warm, shallow water, grow to more than 4 m (13 ft). Hammerheads, tropical water sharks known for their flattened, T-shaped heads, frequently exceed 3.5 m (12 ft), and great white sharks, perhaps the greatest predators of the sea, often reach lengths over 6 m (20 ft). Hard, teethlike scales cover the shark's body surface, giving its skin a sandpaper-like feel. These scales, called placoid scales or dermal denticles, do not get larger as the shark grows. Instead, the animal grows more scales to cover the growing body surface. Most sharks exhibit gray, blue, or brown coloring, shades that blend in well with their marine habitat. Sharks' ventral sides, or undersides, are usually lighter in color than their dorsal, or top, sides. This countershading makes the animals more difficult to spot in the water. When viewed from above, their darker dorsal sides are difficult to distinguish from the ocean depths, and when viewed from below, their lighter ventral sides blend with the sunlit water above them. In most species, the mouth and nostrils are on the ventral side, but in some species they are at the tip of the snout. Sharks are perhaps best known for their short, powerful, hinged jaws and multiple rows of sharp, triangular teeth, which they use to crush or tear pieces of flesh from their prey. Biologists believe that sharks' formidable teeth are actually enlarged, modified placoid scales. A shark's teeth are embedded in its gums, rather than firmly planted in the jaws, and sharks often lose teeth while killing and eating live prey. In most sharks, only the front two rows of teeth are upright and functional, but some species have as many as eight functional rows. Several rows of replacement teeth continually develop behind the teeth a shark actually uses. Developing teeth lie flat, just above the gums, and as teeth are lost, new ones move forward and upright to replace them. When a lemon shark loses a tooth, it takes only eight days for a new one to completely replace it. Some sharks may grow, lose, and replace as many as 30,000 teeth in their lifetime. Just behind the head are five to seven gill slits, which the shark uses to filter oxygen from the water. Unlike most fishes, sharks do not have muscles that draw water across the gills. Although some sharks can pump water through their gills by opening and closing their mouths, most sharks rely on swimming movements to push water across their gills. Some sharks have spiracles, special gill slits just behind the eyes. Spiracles supply oxygen for respiration when a shark eats. Five different kinds of rigid fins supported by cartilage aid sharks in swimming. Depending on the species, sharks may have one or two dorsal fins, which provide stabilization in the water. Sharks also possess two sets of paired fins located on their ventral surface. One set, the pectoral fins, is located just behind the head. Pectoral fins provide lift as sharks move up and down in the water column and stabilization when they swim at a constant depth. They are also used for steering and turning. The second set of paired fins is the pelvic fins. Located toward the tail, these fins help the shark to swim in a level position. In males, modifications to the inner edge of each pelvic fin, called claspers, are used in mating. Some sharks also have an anal fin, a small ventral fin to the rear of the pelvic fins that provides further stability. The last fin is the tail fin, or caudal fin. Often crescent-shaped, the tail fin is the primary source of propulsion for sharks. The heavy tail and body muscles work together with the shark's flexible cartilage skeleton to provide a powerful and smooth swimming stroke. Not all sharks share this general shape and appearance. The angel shark has a flattened body, more like that of a skate, and the wobbegong shark is covered with fleshy flaps of skin that help to camouflage the animal. The hammerhead's unusually shaped head provides lift in the water and enables it to make sharp, darting turns. Nurse sharks have unusually thick skin, which protects them from the sharp corals along the ocean bottom, where they seek protection. A Internal Organs The shark's digestive system is efficient, capable of digesting whole fishes and mammals, and even hard-shelled mollusks and crustaceans. Unlike most animals, which have long, coiled intestines, a shark's intestine is short and straight. To increase intestinal surface area, shark intestines have a special feature called a spiral valve, a region of the intestine lined with membranous folds. The spiral valve functions to slow digestion and increase food absorption. The digestive tract empties into the cloaca, a rear chamber that also receives wastes from the urinary tract and eggs or sperm from the reproductive system. Wastes are eliminated from the body via the anus. A special salt-secreting gland, the rectal gland, helps eliminate excess salts absorbed from food and the marine environment. Unlike their relatives the bony fish, sharks do not have a gas-filled swim bladder to help them float or change depth. Instead, sharks rely on their large, oil-filled livers for buoyancy. A shark's liver is made of two large lobes that concentrate and store oils and fats, substances that are lighter than water. The liver can comprise as much as 25 percent of the shark's total body weight and provides the shark with a small degree of buoyancy while swimming. But even with added help from the liver, a shark must swim continuously or it will sink to the bottom. Sharks have two-chambered hearts that are relatively small compared to the rest of their bodies. Blood flows from the heart to the gills, where it collects oxygen from water and then distributes it to the other organs and tissues. The small heart produces weak blood pressure, and many sharks must swim continuously to create the muscular contractions needed to circulate blood throughout their bodies. Most sharks are cold-blooded--that is, they do not generate heat by digesting food. Instead, the body temperature of most sharks matches the temperature of the water around them. There are, however, some notable exceptions. The body temperatures of fast-swimming sharks, such as blue sharks and makos, can rise almost 10 Celsius degrees (18 Fahrenheit degrees) above the temperature of the surrounding water. When swimming at great speeds, their muscles generate heat, which is transferred between adjacent arteries and veins. B Sensory Systems Sharks' well-developed sensory systems provide them with unmatched advantages when hunting or feeding. Almost one-third of their brain is devoted to the sense of smell. A shark's sense of smell is so powerful that it can detect odors in the water hundreds of meters from their source. Sharks can detect as little as one part per million of substances in the water, such as blood, body fluids, and chemical substances produced by animals under stress. Some sharks can detect as few as ten drops of liquid tuna in the volume of water it takes to fill an average swimming pool. Sharks' eyes detect very small movements and they can see in low-light conditions, making them effective hunters in dark waters. Like cats and other nocturnal hunters, sharks possess a reflective layer in the back of their eyes, called the tapetum lucidum, which magnifies low levels of light. In clear water, sharks see their prey when it is about 20 to 30 m (70 to 100 ft) away. Sharks' eyes also contain cells that detect color, and behavioral studies suggest that sharks can see colors as well as black, white, and shades of gray. These studies also revealed that shiny objects and bright colors, such as yellow and orange, may attract sharks. Sharks use an additional sensory system, called the lateral line, to detect vibrations in the water caused by sounds or animal movements. The lateral line consists of a narrow strip of sensory cells running along the sides of the body and into the head. This sensory system is especially sensitive to sounds in the low-frequency ranges, such as those emitted by struggling wounded fish or other animals. Additionally, sharks sense the weak electrical currents associated with the functioning of nerves and muscles in living animals. The shark's electrosensors, called the ampullae of Lorenzini, are distributed in clusters over its head. This electroreception system is effective only over distances of less than 1 m (3 ft). It may aid sharks in the final stages of feeding or attack. Biologists also agree that this system may somehow enable sharks to detect the weak electromagnetic fields of the Earth, aiding them in migration. III DIET AND FEEDING As a group, sharks will eat almost anything--even other sharks. When examining the stomach contents of dead tiger sharks, for example, biologists have recovered the antlers of a deer, shoes, a driver's license, and even a chicken coop with feathers and bones still inside. But most sharks are more discriminating in their diet. The largest sharks, such as whale sharks and basking sharks, feed on plankton by straining these tiny marine plants and animals from the water. Its sheer size and feeding behavior, similar to that of many large whales, earned the 13-ton, school-bus-sized whale shark its name. The massive fish swims with its mouth open, sucking water filled with plankton, krill, and other tiny organisms into its mouth and through spongy tissue between its gills. After closing its mouth, the shark uses specialized structures called gill rakers to filter its food from the water. Basking sharks cruise slowly along the surface, giving them the appearance of basking in the sun. Actually, they skim the surface to filter shallow-dwelling plankton through their specialized gills. Fish is the primary food of preference for most meat-eating sharks, although they also eat mollusks and crustaceans, such as crabs and shrimps. Sharks will often select weakened or diseased members of a population of prey because they are easier to catch and kill. Some sharks eat carrion, the dead and putrefying flesh of other marine animals. Large, fast sharks, such as great white sharks, may consume seals, dolphins, whales, and other marine mammals. Predatory sharks often circle their prospective prey, sometimes bumping it with their snout or their pectoral fins. Most sharks seize their prey in their powerful hinged jaws. As they bite down, the lower teeth puncture and hold the prey in place, while the upper teeth tear it into pieces. Cookie-cutter sharks and nurse sharks use suction to obtain their meals. The tiny, deep-sea cookie-cutter shark attaches its strong, sucking mouth to tuna, whales, and even larger sharks, and then twists off, slicing out a core of flesh. It swims away to digest its meal, leaving characteristic chunky round holes in its prey, which usually survives. Bottom-feeding nurse sharks suck their prey--bottom-dwelling fish, shrimp, squid, octopus, and shellfish--from holes and crevices on the ocean bottom. IV REPRODUCTION AND DEVELOPMENT Mating behavior in sharks is complex, and very few people have observed it in detail. Biologists know that female sharks mate for only a few weeks during the mating season, which occurs once a year in some species but only once every two or three years in other species. Unlike other fishes, which may produce thousands of offspring each time they reproduce, sharks generally do not reproduce in large numbers. Some species may produce only one baby, or pup, at a time, while others may produce 20 or more. Shark fertilization, when a male's sperm unites with a female's egg, occurs inside the female's body. Male sharks grip females by their pectoral fins before mating begins, perhaps to orient the female in a position conducive to mating. Males of many species may bite females on the pectoral fins or on the back to hold onto them during mating. Biologists have found that in some species that bite during mating, such as the blue shark, the skin on the females' back is twice as thick as it is on the males'. Even so, many female sharks bear scars from the violent bites of courting males. Most sharks mate while swimming side-by-side, but in some of the smaller species, the male may coil himself around the female. During the mating process, the male inserts his clasper, the modified edge of his pelvic fin, into the female's cloaca, forcing sperm down a groove in the clasper and into her body. A female shark may carry a male's sperm in her body for up to a month, enabling fertilization to occur weeks after mating. The gestational period--the time from fertilization to birth--varies from 5 months in nurse sharks to 20 months or longer in spiny dogfish. Because most sharks are cold-blooded, gestation periods can be influenced by water temperature. In warmer seas, development is accelerated, while in colder regions, embryonic development can take much longer. Depending on the species, shark embryos develop in three distinct ways: viviparous, oviparous, and ovoviviparous. Embryonic development in viviparous species, including many larger species such as hammerheads, tiger sharks, and blue sharks, is similar to embryonic development in mammals. Developing embryos are nurtured through a connection to the mother called a yolk sac placenta. When they are sufficiently developed, the pups are born live. Most viviparous species have between 6 and 12 pups at a time. About one-third of all shark species are oviparous, including wobbegongs and horn sharks. In these species, fertilized embryos develop in eggs outside of the female's body. The female secretes a protective shell or case around the embryo and then deposits it in the sea. After a short time the leathery shell hardens and serves to protect the embryo, sustaining it with nutrients in the yolk, until the embryo hatches. The eggs are often equipped with tendrils that anchor them to rocks or weeds. Ovoviviparous development is the most common type of embryonic development in sharks. In ovoviviparous species, such as great whites and sand tiger sharks (called gray nurse sharks in Australia), the female secretes a thin protective case around the embryo, but the embryo remains in one of her two uteri to develop. Each uterus may contain several developing embryos. When they are sufficiently mature, the eggs hatch, still inside the female. The female then gives birth to live pups. Ovoviviparous sharks differ from viviparous sharks because there is no placental connection between the mother and the embryo. Instead, the embryo lives on nutrients in its yolk sac. In ovoviviparous species, embryos may derive additional nutrients by swallowing unfertilized eggs in the mother's uterus. In rarer cases, developing embryos eat other developing embryos in the uterus. This practice, called intrauterine cannibalism, is seen in sand tiger sharks, as well as in other species. Whether shark pups hatch from eggs or from their mother's uterus, they are ready to feed and survive on their own immediately after birth. Parental behavior has never been observed in sharks. In fact, as soon as they are born, pups of many species swim away from their mothers, perhaps to avoid being eaten. Most sharks grow very slowly and do not mature sexually until late in their lives. Although little is known about shark life spans in the wild, some biologists estimate that the average shark life span is about 25 years. Longer-lived sharks, such as the spiny dogfish, may live 70 to 100 years. Some scientists suspect that other species may live even longer. V BEHAVIOR Sharks, like other fishes, sometimes congregate in large groups. This schooling behavior is thought to aid in protection, reproduction, and feeding. In some species, such as reef whitetip sharks, the animals gather by the hundreds in shallow coves and lagoons, where they enter a state of lethargy that biologists have compared to napping. Nurse shark males have been observed traveling in groups and appear to participate in group mating activities. Biologists do not know if other shark species exhibit similar behavior. Frequently, large groups of sharks demonstrate violent, uncontrolled feeding behaviors called feeding frenzies. Biologists believe feeding frenzies occur when large amounts of blood or food in the water confuse the sharks' sensory systems. Frenzied sharks attack everything that moves, even each other. They ferociously snap and slash about, creating more sensory stimuli and attracting more sharks to the group. This behavior intensifies as the group gets larger and larger, encompassing sharks from many different species. Humans may whip sharks into a feeding frenzy for observation or fishing purposes by dropping chum--blood and bits of dead fish and other meat--into the water. Such activities, while instrumental in attracting sharks for research or fishing, have undoubtedly contributed to the sharks' undeserved reputation as hungry predators ever in search of a human meal. Although sharks are widely considered a threat to humans, no sharks naturally feed on humans, and less than 10 percent of the 375 known species have actually been implicated in attacks. Most researchers agree that attacks on humans are often a mistake on the shark's part. Researchers believe that great white sharks, infamous for their apparent willingness to attack humans, attack humans because they mistake them for large sea mammals, such as seals or sea lions. In other instances, sharks attack when humans provoke them by getting too close to the shark or the shark's food. The International Shark Attack File, maintained by the Florida Museum of Natural History, compiles statistics from around the world and shows that the number of human attacks worldwide averages less than 100 per year, and very few of those are fatal. Considering the sheer number of people each year who enter the sharks' marine environment for recreational purposes--such as swimming, diving, water skiing, and boating--these numbers are quite low. In fact, a person has a better chance of dying from choking on their own dinner than they do of dying from a shark attack. VI CONSERVATION AND RESEARCH Humans have hunted sharks for sport, food, medicine, and leather for centuries, with little regard for the health of shark populations. Sports fishers around the world regard sharks as some of the most challenging fish to catch in the sea. Shark flesh is highly prized in many regions of the world. One particularly popular food made from shark meat, shark fin soup, is in such demand that some fishers hunt sharks just for their fins, throwing the rest of the fish back to the sea to die. Shark liver oil is a popular source of vitamin A, and some people believe that shark liver and cartilage are beneficial to human health. Shark skin, with its microscopic teethlike scales, was once used as a fine grade of sandpaper, and when the scales are removed from the skin to make shark leather, it brings high prices for use in shoes, belts, and handbags. Many sharks are killed unintentionally. Each year, thousands of sharks die in nets set out to catch other types of fish. Sometimes, humans kill sharks just because they fear them. Such activities have placed many shark populations in danger of extinction. For example, between the mid-1980s and mid-1990s, populations of dusky sharks and sandbar sharks off the eastern coast of the United States declined by more than 80 percent. Internationally, the sand tiger shark and the great white shark are also in danger of extinction. Sharks grow slowly, reproduce late in their lives, and produce few offspring when they mate, making the natural rate of population replenishment very slow. If too many sharks in a particular area are killed, that population may never recover. For example, numbers of porbeagle sharks, swift, ocean-going sharks once commercially valuable, declined dramatically until, by 1960, commercial fishers could no longer catch enough of them to cover their expenses. Thirty years later, porbeagle populations still have not recovered. As researchers begin to better understand the devastation human activities have brought to many shark populations, they also better understand the benefits sharks provide. Losing these top-level predators produces devastating effects to local ecosystems. Moreover, medical researchers are interested in learning more about sharks, particularly their immune systems. Sharks recover rapidly from severe injuries. They appear to be nearly impervious to infection, cancers, and circulatory diseases. For decades some people have believed that shark cartilage has anticancer properties. Although recent scientific studies challenge this belief, medical researchers continue to investigate the shark immune system in hopes of one day applying its secrets to fight human disease. To prevent the effects of overfishing and other human activities in the United States, state and federal management plans restrict the number of sharks that can be legally killed. Plans also require that fishers take entire sharks, instead of just the fins, preventing the practice of cutting off the fins and leaving the rest of the fish to waste. In many countries, including South Africa, Australia, and the United States, legislation specifically protects great white sharks, a species widely prized as a game fish and considered to be endangered in many areas where they once roamed in large numbers. While nets around bathing beaches prevent sharks from entering popular waters, such nets claim the lives of thousands of sharks each year. More sophisticated methods to repel sharks are under investigation. Chemical substances similar to fish toxins have been developed, and the repellent effects of many detergents may offer a chemical means to deter shark attacks. Electrical devices that interfere with shark sensory systems may one day offer another alternative to nets. Scientific classification: Sharks belong to the class Chondrichthyes and the subclass Elasmobranchii. The estimated 375 species are divided into 8 orders and 30 families. The bull shark, blue shark, tiger shark, common lemon shark, dusky shark, sandbar shark, and the reef white tip shark belong to the requiem family, Carcharhinidae, and are classified as Carcharhinus leucas, Prionace glauca, Galeocerdo cuvier, Negaprion brevirostris, Carcharhinus obscurus, Carcharhinus plumbeus, and Triaeonodon obesus, respectively. The great white shark, mako shark, and porbeagle belong to the mackerel shark family, Lamnidae, and are classified as Carcharodon carcarias, Isurus glauca, and Lamna nasus, respectively. Dwarf dogfish and spiny dogfish belong to the dogfish family, Squalidae, and are classified as Etmopterus perryi and Squalus acanthias. The great hammerhead shark belongs to the hammerhead family, Sphyrnidae, and is classified as Sphyrna mokarran. The whale shark is the only member of the whale shark family, Rhiniodontidae, and is classified as Rhiniodon typus. The nurse shark belongs to the nurse shark family, Ginglymostomatidae, and is classified as Ginglymostoma cirratum. The sand tiger shark belongs to the sand tiger shark family, Odontaspididae, and is classified as Carcharias taurus. Wobbegong sharks make up the wobbegong family, Orectolobidae, and is classified as Orectolobus ornatus. The Pacific angel shark belongs to the angel shark family, Squatinidae, and is classified as Squatina californica. The thresher shark belongs to the thresher shark family, Alopiidae, and is classified as Alopias vulpinus. The sixgill shark belongs to the six-gill and seven-gill shark family, Hexanchiformes, and is classified as Hexanchus gigas. The basking shark belongs to the basking shark family, Cetorhinidae, and is classified as Cetorhinus maximus. The megamouth shark makes up its own family, Megachasmidae, and is classified as Megachasma pelagios. The cookie-cutter shark is a member of the luminous shark family, Dalatiidae, and is classified as Isistius brasiliensis. The horn shark belongs to the horn shark family, Heterondontidae, and is classified as Heterodontus francisci. Contributed By: Jeffrey C. Carrier Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.
